B. DETERMINATION OF THE LEAD. i. ElectrolyticaUy. The lead and
iron sulphides (see p. 255) are washed with water containing a little ammo-
nium or sodium sulphide, and the filter and precipitate heated to boiling
with 15-20 c.c. of nitric acid (D 1-2) in a small covered beaker. The solution
is decanted through a filter and the residue boiled with 20-30 c.c. of the
following mixture, which dissolves also any small amount of lead sulphate
formed by oxidation of the sulphide : 40 c.c. of ammonia (D 0-923), 67 c.c.
of nitric acid (D 1-332) and 5 grams of copper nitrate. The liquid is filtered,
the filter washed with hot water acidified with nitric acid, and the filtrate
made up to about 200 c.c. and electrolysed to determine the lead : Winkler
cathode; matte gauze cylinder anode ; ND = 0-3 amp. ; voltage, 1-8-2
(see also Electrolytic Determination of Lead in Ordinary Brass).
When the deposition of the lead is complete, the anode is washed with
water alone, dried at 200-220° and weighed : Pb02 X 0-866 = Pb.
2. Gravimetricatty. The lead and iron sulphides (see I, p. 255) on the
filter are well washed with water containing a little ammonium sulphide,
dissolved in nitric acid (D 1-2) and filtered by decantation. The residue
is again boiled twice with nitric acid and finally washed with water acidified
with nitric acid. The nitric acid solution is evaporated with sulphuric acid
until white fumes appear, and the residue, when cold, taken up in water,
the lead being determined as sulphate as in the case of tin.
2. Determination of the Lead alone.—The surface of the tin-plate
is scraped with a penknife so that as little as possible of the metal under-
neath is removed, about 1-1-5 gram of thin shavings being obtained ; any
particles of iron are removed with a magnet. The shavings are mixed in
a covered beaker with 6 c.c. of fuming nitric acid (D 1-5), 3 c.c. of water
being poured carefully down the side of the beaker so that the two liquids
mix slowly and the action proceeds regularly. At the end of the action,
the liquid is heated to boiling, diluted with 50 c.c. of boiling water and,
when the liquid above the metastannic acid has become clear, filtered by
decantation through a dense filter and washed by decantation with water
slightly acidified with nitric acid.
In the filtrate the lead is determined either electrolytically or gravi-
metricaUy by the methods given for the analysis of alloys of lead and tin
(see p. 259). When the lead is separated as peroxide or sulphate, the iron
unavoidably present is determined in the residual liquid by precipitating
with ammonium chloride and ammonia. Deduction of the weight of iron
found from the total weight of the shavings gives the amount of lead-tin
alloy taken for analysis, and to this the quantity of lead found is referred.
Good tin-plate should exhibit a clean, smooth surface without spots or
bubbles and with a uniformly thick layer of tin. It should have at least 0-25-0-3
gram of tin per sq. dm. (the best qualities have 0-6-0-7 gram per sq. dm.), and
the tin used should not contain more than i% of lead. subsequent tinning, the